The invention relates to a self-cleaning, air-tight centrifugal separator in which the connection of the rotating drum chamber to the stationary discharge fitting is sealed by means of a flange packing. The flange packing, which is tightly clamped in the stationary discharge fitting surrounding the drum, has an annular flange of U-shaped cross-sectional profile, which is urged tightly against the neck of the drum by the pressure of a liquid. A centrifugal separator of this kind is described, for example, in German Pat. No. 1,094,193.
Centrifugal separators of the air-tight type are used mostly for the processing of liquids which must be preserved against contact with air so as to prevent oxidation.
Due to the airtight packing, the drums of separators of this type fill up completely with liquid shortly after they are started up, because the air that is in them is quickly expelled by the incoming liquid. As operation continues, then, contact with the atmospheric air is excluded.
In the beverage industry, self-cleaning, air-tight centrifugal separators are operated in most cases with intermittent partial ejection of the solids, that is to say, at relatively short intervals of time, e.g., every three minutes, only a portion of the solids separated from the liquid is ejected, while the remainder is retained in the drum as a safety seal to prevent the leakage of valuable liquid. Therefore, during such partial ejections there is no need to shut off the raw liquid feed, as there is when solids are ejected all at once.
The speed of the ejection of the solids, disregarding internal friction, is equal to the circumferential speed of the drum, so that in only one second a relatively large amount of the solids accumulated in the drum is ejected, whereupon an airless void is formed in the central part of an air-tight drum, producing a corresponding suction effect.
Let it be assumed that a separator is fed with raw liquid at a rate of 24,000 l/hr, i.e., at 6.66 l/sec, and when a partial ejection is performed 18 liters of solids are ejected in one second. An airless void then forms in the central part of the drum, whose volume is 18 l, minus the amount of liquid entering the drum in the meantime; the latter amount, however, due to the suction effect, will be greater than 6.66 l, unless a volumetric pump is installed in the feed line. If the overfeed amounts to 10 l/sec, there will still remain an airless void of 8 liters seeking equilibrium with the atmosphere.
With regard to the pumping of the clarified liquid through the discharge line, there are essentially two types of construction.
In the one type of construction, the feed pump forces the raw liquid through the drum and the discharge line. The liquid is thus under pressure throughout, and also forces the flange of the packing tightly against the neck of the drum. But when an airless void is formed in the drum during the partial ejection of solids, the pumping pressure can no longer propagate itself all the way into the discharge line, and the discharge line thus loses pressure, whereupon the packing flange lifts away from the neck of the drum so that air is sucked into the drum. The fact that in this case a portion of the clarified liquid may be sucked back into the drum can be disregarded.
In the other type of construction, there is provided in the stationary discharge fitting a special chamber in which a centrifugal pump impeller rotates with the drum. The feed pump forces the liquid all the way into this chamber, from which it is then pumped by the centrifugal pump impeller. When an airless void forms during the partial ejection of solids, the pressure is removed from the packing flange and the flange is lifted away from the drum neck so that, in this case, too, air can be drawn into the drum. The difference between this and the first type of construction described is that in this case clarified liquid cannot be sucked back from the discharge line.
The density of air at atmospheric pressure is 1.29 mg. .times. cm.sup..sup.-3, and its oxygen content is 21%. If 8 l of air is sucked into the drum during a partial ejection of the solids, this corresponds to a quantity of 2167.2 mg of oxygen. At the throughput of 24,000 l/h mentioned in the beginning, 1,200 l of liquid flows through the drum in a centrifuging period of 3 minutes. The air that is sucked in mixes with this amount of liquid, so that the oxygen content of the liquid amounts to 1.8 mg/l. Breweries, however, strive for an oxygen content of less than 0.15 mg/l, which cannot be achieved under the conditions of the types of construction known hitherto. However, with smaller ejections of solids and longer centrifuging periods the above requirement can largely be satisfied.
The invention is addressed to the problem of designing a centrifugal separator of the kind described, in such a manner that no aspiration of atmospheric air will be possible during the partial ejections of solids. This is accomplished in accordance with the invention by the fact that a liquid seal is provided, which can maintain at least an equilibrium with an external excess pressure of 1 atmosphere.
Liquid seals are known in connection with non-air-tight centrifugal separators. For example, British Pat. No. 675,648 shows a chamber located above the paring chamber and rotating with the drum, this chamber being filled at least partially with liquid in which a stationary disk is immersed. In this manner, outside air is prevented from entering the paring chamber and thus from reaching the passages in the paring disk.
In this known system, virtually the same pressure prevails in the sealing chamber above and below the immersed disk. Its purpose, therefore, is not to seal a vacuum in the drum chamber against the atmospheric pressure, because in this known drum no vacuum can form in the drum chamber. The sealing chamber is also independent of the clarified liquid circuit.